Structural transition and possible pressure-induced superconductivity in a suboxide <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>La</mml:mi> <mml:mn>5</mml:mn> </mml:msub> <mml:msub> <mml:mi>Pb</mml:mi> <mml

Here we report a structural phase transition and its interplay with superconductivity in the suboxide ${\mathrm{La}}_{5}{\mathrm{Pb}}_{3}\mathrm{O}$. Upon cooling through ${T}_{t}=225\phantom{\rule{0.28em}{0ex}}\mathrm{K},{\mathrm{La}}_{5}{\mathrm{Pb}}_{3}\mathrm{O}$ transforms from a high-temperature $I4$/mcm to a low-temperature $P4$/ncc structure in which La-Pb dimerization along the $c$ axis occurs. This transition is accompanied by anomalies in the temperature dependence of electrical resistivity and specific heat. High-pressure electrical transport measurements reveal that hydrostatic pressure suppresses the structural transition and possibly induces superconductivity with a maximum superconducting temperature of 10 K. Density functional theory calculations show minimal changes in the electronic density of states and no gap opening at ${E}_{F}$ across ${T}_{t}$, suggesting that the transition is driven by bonding effects rather than Fermi surface instability. These findings establish ${\mathrm{La}}_{5}{\mathrm{Pb}}_{3}\mathrm{O}$ as a promising platform for exploring the interplay between weak structural transitions and superconductivity.